An electric power source ionizes hydrogen, deuterium, or helium fuel into a plasma by stripping away electrons. Magnetic fields then direct the charged gas in the proper direction to provide thrust.

“A rocket engine is a canister holding high-pressure gas,” Chang Diaz explained. “When you open a hole at one end, the gas squirts out and the rocket goes the other way. The hotter the stuff in the canister, the higher the speed it escapes and the faster the rocket goes. But if it’s too hot, it melts the canister.”

The VASIMR engine is different, Chang Diaz explained, because of the fuel’s electrical charge: “When gas gets above 10,000 [kelvins], it changes to plasma – an electrically charged soup of particles. And these particles can be held together by a magnetic field. The magnetic field becomes the canister, and there is no limit to how hot you can make the plasma.”

Chang Diaz has pointed out that hydrogen would be an advantageous fuel for the VASIMR engine because the spacecraft would not have to lift off carrying all the fuel it needs for the journey.

VASIMR® System. Image Credit: Ad Astra Rocket Company

“We’re likely to find hydrogen pretty much anywhere we go in the Solar System,” he said.

A spacecraft using conventional chemical rockets would take eight months to get to Mars during opposition. However, the VASIMR engine would make the journey in as little as 39 days.

Chang Diaz explained: “Remember, you are accelerating the first half of the journey – the other half you’re slowing, so you will reach Mars but not pass it. The top speed with respect to the Sun would be about 32 miles per second [or 51.5 km/s]. But that requires a nuclear power source to heat the plasma to the proper temperature.”

The use of nuclear power in space is not without its controversy. In 1997, there was widespread public concern when NASA’s Cassini probe, which carried a plutonium battery, made a flyby of Earth to perform a gravity assist. Although NASA denied that the risk to the public, should an accident occur, was no greater than that posed every day by other sources of radiation, some scientists, including the popular theoretical physicist Michio Kaku, disagreed.

In April 1970, the Atomic Energy Commission was deeply concerned about the return of Apollo 13 to Earth. Where an Apollo mission would usually leave the lunar module’s descent stage on the Moon, the unsuccessful Apollo 13 dropped its lunar module Aquarius, with its plutonium-powered scientific experiments, into the ocean, raising concerns about radioactive contamination.

Elon Musk, CEO of Space Exploration Technologies Corporation (SpaceX), is skeptical about the viability of the VASIMR engine. One reason is the concern about radioactive debris falling to Earth in the event of an accident.

Musk is also skeptical that the VASIMR engine would be a significant improvement over chemical rockets, stating: “So people like Franklin – basically it’s a very interesting ion engine he’s got there, but it requires a big nuclear reactor. The ion engine is going to help a little bit, but not a lot in the absence of a big nuclear reactor.” Musk also points out that the big nuclear reactor would add a lot of weight to a rocket.

Chang Diaz dismisses the concerns about nuclear reactors in space, stating: “People are afraid of nuclear power. Chernobyl, Three Mile Island, Fukushima – it is a little misunderstood. But if humans are truly going to explore space, we eventually will have to come to grips with the concept.”

Another vocal critic of the VASIMR engine is Robert Zubrin, president of The Mars Society, who designed the Mars Direct plan to colonize Mars and wrote the popular book The Case For Mars. He has gone as far as to call the VASIMR engine a “hoax”.

Zubrin wrote in SpaceNews: “To achieve his much-repeated claim that VASIMR could enable a 39-day one-way transit to Mars, Chang Diaz posits a nuclear reactor system with a power of 200,000 kilowatts and a power-to-mass ratio of 1,000 watts per kilogram. In fact, the largest space nuclear reactor ever built, the Soviet[-era] Topaz, had a power of 10 kilowatts and a power-to-mass ratio of 10 watts per kilogram. There is thus no basis whatsoever for believing in the feasibility of Chang Diaz’s fantasy power system.”

Chang Diaz, however, says in his paper: “Assuming advanced technologies [emphasis added] that reduce the total specific mass to less than 2 kg/kW, trip times of less than 60 days will be possible with 200 MW of electrical power. One-way trips to Mars lasting less than 39 days are even conceivable using 200 MW of power if technological advances allow the specific mass to be reduced to near or below 1 kg/kW.”

In other words, Chang Diaz is allowing for further developments that would enable such a reactor.

Zubrin, however, stated: “[T]he fact that the [Obama] administration is not making an effort to develop a space nuclear reactor of any kind, let alone the gigantic super-advanced one needed for the VASIMR hyper drive, demonstrates that the program is being conducted on false premises.”

Whether the VASIMR engine is viable or not, in 2015, NASA awarded Chang Diaz’s firm – Ad Astra Rocket Company™ – a three-year, $9 million contract. Up to now, the VASIMR engine has fired at fifty kilowatts for one minute – still a long way from Chang Diaz’s goal of 200 megawatts.

In its current form, the VASIMR engine uses argon for fuel. The first stage of the rocket heats the argon to plasma and injects it into the booster. There, a radio frequency excites the ions in a process called ion cyclotron resonance heating. As they pick up energy, they are spun into a stream of superheated plasma and accelerated out the back of the rocket.

Collin R. Skocik has been captivated by space flight since the maiden flight of space shuttle Columbia in April of 1981. He frequently attends events hosted by the Astronaut Scholarship Foundation, and has met many astronauts in his experiences at Kennedy Space Center. He is a prolific author of science fiction as well as science and space-related articles.

In addition to the Voyage Into the Unknown series, he has also written the short story collection The Future Lives!, the science fiction novel Dreams of the Stars, and the disaster novel The Sunburst Fire. His first print sale was Asteroid Eternia in Encounters magazine. When he is not writing, he provides closed-captioning for the hearing impaired. He lives in Atlantic Beach, Florida.

Current ion engines use an electric field to accelerate the ions, but don’t contain the ions in a magnetic field. There’s a screen at the exit of the thruster that the ions race by, and there has to be a balance between the ions’ energy and the decay of that screen. By constricting the flow with a magnetic field, there’s no contact with anything physical, and the ions’ energy can be cranked way up increasing thrust. That said, the amount of energy required is still very high, and the arguments about large nuclear reactors being required are still valid.

The VASIMR engine is a legitimate propulsion device with excellent high power (>200 kW) performance that out performs ion and hall engines in that power regime. The issue has always been the power source. Solar is up and coming and nuclear like it or not is required to explore the solar system. There’s actually very little risk with nuclear with the exception of political risk. Until you turn on a fission reactor it hasn’t built up any nuclear waste. It is simple, don’t turn on the reactor until you are far away from Earth. Such technology is required for human missions.

Please read more closely – the call is for 200MW or 200,000kW. The dimensions called for would be at least 50 years away in needed research, advancements Chang-Diaz is assuming, and development cycles.

Doable with solar concentrators: mirrors with large collecting area that focus down the light to smaller area solar cells. Reflecting surfaces (in space) are light, solar cells are heavy.
Getting reflecting surface that size to space is not that difficult. See for example the Sunjammer solar sail mission:

If I understood this correctly, the exhaust doesn’t make contact with any surfaces. Wouldn’t that mean a long life cycle at relatively high thrust levels compared to other ion thrusters? That to me is the greatest benefit and well worth the investment which is tiny. A VASIMR powered drone could spend years clearing up the mess we’ve made in orbit or be used as a cargo hauler between Earth and the Moon. For this use, I’m guessing you could probably get away with 200-300kw of solar power (recent NASA sponsired solar panel research suggests this is coming in the not too distant future) so the nuclear reactor issue is really just icing on the cake. If it becomes possible then great, if not there’ll still be plenty of useful applications for a functional VASIMR system at the targeted (by NASA) power levels that are better suited to this technology than other ion thrusters. Also by 2068 we may very well have fusion reactors to do this which could be much smaller than their fission counterparts and won’t have the same worldwide objections.

Another use for the VASIMR that I didn’t consider before is as propulsion for asteroid mining vehicles/mineral transporters. In terms of our presence beyond LEO, this is probably one of the most important potential ventures. Extracting and refining raw materials in space would produce the basis for an orbital construction infrastructure. Something like this ensures continued presence, not just expensive one off shots in the dark. VASIMR could potentially do this way better than rockets can (fuel efficiency) and it looks to have greater thrust capabilities than traditional ion technology. I have no doubt that it’s not perfect but suggesting this technology should get us directly to Mars and if it doesn’t it’s basically not an improvement or the people making it are lying to get cash is not really the way to go in my opinion.

Much as I tend to agree with Zubrin, I also recognize that he can a mite prickly on any topic which smacks of interposing sizable technological or programmatic obstacles in the path toward speedy Mars missions. I can see how he would balk at the idea of inventing a whole new drive system to get there, as it could be another show-stopping delay. Same might go for Musk.

Still, the advantages of such a drive system in terms of ISp and thus of time/payload improvements can’t be brushed off. As for power, a LFTR system could be significantly smaller (and thus more feasible) than even a highly advanced, miniaturized HPLWR type system. LFTRs are also meltdown-proof and require less refined fuel (and produce fewer trans-uranic isotopes as ‘waste’).